Linear polymers of carbon monoxide with ethene, in which polymers the units from carbon monoxide and the units from ethene are present in a substantially alternating arrangement can be prepared by contacting the monomers at elevated temperature and pressure with a catalyst composition containing a Group VIII metal and a phosphorus bidentate ligand of the general formula (R.sub.1).sub.2 P-R-P(R.sub.1).sub.2 in which R.sub.1 represents an optionally polar substituted monovalent aromatic hydrocarbon group and R is a divalent organic bridging group. In regard to this bridging group, there is preference for a group which in the bridge connecting the two phosphorus atoms with each other contains three atoms, of which at least two are carbon atoms and among which no two atoms are present which together form part of a single cyclic structure. It has been found that phosphorus bidentate ligands in which such a bridge occurs give the catalyst compositions the highest polymerization rate. An example of such a phosphorus bidentate ligand is 1,3-bis(diphenylphosphino)propane.
A drawback of the alternating carbon monoxide/ethene copolymers is that they have a very high melting point. The processing of these polymers should take place in the molten state with the material being processed at a temperature which is at least 25.degree. C. above the melting point. It has been found that these polymers are not particularly resistant to the high temperatures required in their processing and, as a result, discoloration and decomposition can take place. Previously it has been found that the melting point of these polymers could be considerably reduced by incorporating in the starting monomer mixture a relatively small quantity of one or more .alpha.-olefins with at least three carbon atoms per molecule (hereinafter designated as C.sub.3+ .alpha.-olefins). The more C.sub.3+ .alpha.-olefins incorporated in the monomer mixture, the lower the melting point of the polymers obtained. It was also found that the previously mentioned catalyst compositions containing phosphorus bidentate ligand with three atoms in the bridge which exhibited the highest polymerization rate in the polymerization of carbon monoxide with ethene also afforded the highest polymerization rate in the polymerization of carbon monoxide with ethene and additionally one or more C.sub.3+ .alpha.-olefins.
The applicants recently carried out an investigation in order to find out whether, with the use of the above-mentioned catalyst compositions, linear polymers of carbon monoxide with one or more C.sub.3+ .alpha.-olefins (i.e. without ethene) could be prepared in which the units from carbon monoxide on the one hand and the units from the C.sub.3+ .alpha.-olefins used on the other hand occurred in a substantially alternating manner. It was found that such polymers could indeed be prepared in this way, but that the catalyst compositions exhibited only a low polymerization rate in comparison with their previously observed rate in the polymerization of carbon monoxide with ethene and optionally also with one or more C.sub.3+ .alpha.-olefins. Consistent with previous observations in the polymerization of carbon monoxide with ethene, alone or in admixture with one or more C.sub.3+ .alpha.-olefins, the highest polymerization rates in the polymerization of carbon monoxide with one or more C.sub.3+ .alpha.-olefins were obtained using catalyst compositions containing a phosphorus bidentate ligand with three atoms in the bridge. In the course of continued research by the applicants into this subject, it was found that the rate of polymerization of carbon monoxide with one or more C.sub.3+ .alpha.-olefins could be increased if the catalyst composition was modified by replacing the optionally polar substituted monovalent aromatic R.sub.1 hydrocarbon groups in the phosphorus bidentate ligand by optionally polar substituted monovalent aliphatic R.sub.2 hydrocarbon groups. In this modified catalyst composition, the applicants mainly used phosphorus bidentate ligands in which the bridge contained three atoms, since from previous work (see above), it was assumed that this would enable the highest polymerization rate to be achieved with these catalyst compositions. An example of a phosphorus bidentate ligand of this type with the general formula (R.sub.2).sub.2 P-R-P(R.sub.2).sub.2 is 1,3-bis(di-n-butylphosphino)propane.
In the subsequent research into this subject, it has now been surprisingly found that the polymerization rate of the last-mentioned catalyst compositions for the polymerization of carbon monoxide with one or more C.sub.3+ .alpha.-olefins can be raised by replacing the bridging group R in the phosphorus bidentate ligand with a divalent organic bridging group R.sub.3 which contains in the bridge linking the two phosphorus atoms with each other, four atoms, of which at least two are carbon atoms and among which no two atoms are present which together form part of a single cyclic structure. It was further found that an increase of the polymerization rate of the catalyst compositions obtained by replacing therein a phosphorus bidentate ligand with the general formula (R.sub.2).sub.2 P-R-P(R.sub.2).sub.2 by a phosphorus bidentate ligand with the general formula (R.sub.2).sub.2 P-R.sub.3 -P(R.sub.2).sub.2 also occurs in the polymerization of carbon monoxide with ethene in admixture with one or more C.sub.3+ .alpha.-olefins. In complete contrast with this it has been found that a replacement of this kind in the polymerization of carbon monoxide with ethene leads to a decrease in the polymerization rate. Finally it was found that the polymerization rate of catalyst compositions containing a phosphorus bidentate ligand with the general formula (R.sub.2).sub.2 P-R.sub.3 -P(R.sub.2).sub.2 in the polymerization of carbon monoxide with one or more C.sub.3+ .alpha.-olefins and optionally also with ethene can be raised further by replacing therein, at each of the two phosphorus atoms, one of the R.sub.2 groups by an optionally polar substituted aliphatic hydrocarbon group differing in carbon number from R.sub.2. Corresponding with the previously observed deviant behavior of the catalyst compositions containing a phosphorus bidentate ligand with the general formula (R.sub.2).sub.2 P-R.sub.3 -P(R.sub.2).sub.2 in the polymerization of carbon monoxide with ethene, it was now also found in this polymerization that a replacement of this kind led to a decrease in the polymerization rate. Evidently, the favorable effects on the polymerization rate obtained with the above-described modifications of the phosphorus bidentate ligand are only obtained if the catalyst composition is used for polymerizing a monomer mixture containing C.sub.3+ .alpha.-olefins.